Micromachined gyroscopic sensor with detection in the plane of the machined wafer

ABSTRACT

The invention relates to a microgyroscope, that is to say an inertial micromechanical sensor dedicated to the measurement of angular velocities, which is produced by micromachining techniques on a silicon wafer. The gyroscope comprises two symmetrical moving assemblies coupled via a coupling structure. Each of the two assemblies comprises a moving mass surrounded by a moving intermediate frame. The frame is connected to the coupling structure and can vibrate in two degrees of freedom in orthogonal directions Ox and Oy in the plane of the wafer. The mass is connected on one side to the frame and on the other side to fixed anchoring regions via linking means that allow the vibration movement along the Oy direction to be transmitted to the mass without permitting movement of the mass along the Ox direction. An excitation structure is associated with the frame in order to excite its vibration along Ox. A movement detection structure is associated with the mass in order to detect its vibration along Oy.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application is based on International Application No.PCT/EP2003/050785, filed on Nov. 3, 2003, which in turn corresponds toFR 02/13835 filed on Nov. 5, 2002, and priority is hereby claimed under35 USC §119 based on these applications. Each of these applications arehereby incorporated by reference in their entirety into the presentapplication.

FIELD OF THE INVENTION

The invention relates to a microgyroscope, that is to say an inertialmicromechanical sensor dedicated to the measurement of angularvelocities, which is produced by micromachining techniques.

BACKGROUND OF THE INVENTION

The inspiration for micromachining sensors comes from techniques forproducing integrated circuits. This consists in producing, collectivelyon a single thin wafer (in principle a silicon wafer), several tens orhundreds of identical sensors using deposition, doping and photoetchingtechniques that define not only the electrical parts of the sensor butalso the cut-out geometrical features that give the sensor itsmechanical properties.

The etching techniques are well controlled and collective fabricationconsiderably reduces the costs. The robustness of the devices isexcellent and the small size of the structures is highly advantageous.

SUMMARY OF THE INVENTION

To produce a microgyroscope, a suspended vibrating mass is formed in asilicon wafer together with an electrical excitation structure formaking this mass vibrate in a defined direction. When the gyroscoperotates about an axis called the sensitive axis of the gyroscope,perpendicular to this vibration direction, a coriolis force is exertedon the mass. This coriolis force, which is a vector sum of the vibrationmovement and the rotation movement, produces a vibration of the mass ina direction perpendicular both to the excitation vibration and to theaxis of rotation. This resulting natural vibration is detected by adetection structure, which is itself produced by micromachining.

Structures having two vibrating masses that are mechanically coupled inthe manner of a tuning fork have already been produced. The two massesare coplanar and machined in the same silicon wafer.

In general, the sensitive axis of these gyroscopes lies in the plane ofthe silicon wafer and the detection structure detects any movementperpendicular to the plane of the masses using electrodes placed aboveeach moving mass. The electrical signals resulting from this detectionare used to determine an angular velocity of rotation of the gyroscopeabout its sensitive axis.

However, to produce structures for detecting movements perpendicular tothe plane of the moving masses generally requires the gyroscope tocomprise several machined wafers, which have to be joined together. Oneof the wafers includes the actual micromachined vibrating structure withits moving masses, its linking arms and a vibration excitationstructure, while at least one other wafer includes electrodes fordetecting the vibration generated by the coriolis force. To fabricatethe multi-wafer assembly is expensive.

This is why there is also a need to produce technologically simplerstructures, machined in a single silicon wafer, in which an excitationmovement of the moving mass is generated in a direction Ox in the plane,whereas a movement resulting from the coriolis force is detected in adirection Oy in the same plane, perpendicular to Ox. The sensitive axisof the microgyroscope is in this case an Oz axis perpendicular to theplane of the silicon wafer. The excitation structure and the detectionstructure are interdigitated capacitive combs produced when machiningthe silicon wafer. All the electrical structures are produced on thesame wafer as the vibrating mechanical structure. Fabrication istherefore much less expensive.

However, in this type of gyroscope it is necessary for the excitationmovement along the Ox axis to be well separated from the detectionmovement along the Oy axis—specifically, this means that the detectionstructure must detect mainly the movement along Oy that results from thecoriolis force, without the measurement being contaminated by parasiticdetection of the excitation movement along Ox.

One object of the invention is to propose a microgyroscope structurethat allows rotation measurement with a very high sensitivity and verygood linearity, but with minimal perturbations due to the excitationmovement or to other effects. Another object is to propose amicrogyroscope structure that can accommodate, apart from the electricalstructures for inducing vibration and for detecting movement, auxiliaryelectrical structures for adjusting the frequency and for compensatingfor any bias (an angular velocity measurement not equal to zero when theangular velocity is equal to zero) due to intrinsic defects or to thespread in characteristics resulting from mass production.

According to the invention, what is proposed is a gyroscope with avibrating structure, produced by micromachining a thin flat wafer, thisgyroscope comprising two symmetrical moving assemblies coupled by acoupling structure that connects these two assemblies in order to allowmechanical vibration energy to be transferred between them, thisgyroscope being characterized in that each of the two symmetrical movingassemblies comprises two moving elements, an inertial first movingelement being connected to the coupling structure and able to vibrate intwo degrees of freedom in orthogonal directions Ox and Oy of the planeof the wafer, and a second moving element being connected, on one side,to the first element and, on the other side, to fixed anchoring regionsvia linking means that allow the vibration movement of the first elementin the Oy direction to be transmitted to the second element withoutpermitting any movement of this second element in the Ox direction, anexcitation structure being associated with the inertial first movingelement in order to excite a vibration of this element along Ox, and amovement detection structure being associated with the second movingelement in order to detect a vibration of the second element along Oy,the first moving element being a rectangular intermediate framesurrounding the second moving element, denoted by the name moving mass,and the coupling structure comprising two outer frames, each of whichsurrounds the intermediate frame of a respective moving assembly.

In other words, the first moving element is excited into movement alongOx but it does not cause the second element to move in this direction.The first element experiences the coriolis force along Oy (when themicrogyroscope is rotating about a sensitive axis Oz perpendicular to Oxand Oy) and causes the second element to move in this direction thanksto the linking means between these two elements. The coupling structureallows the vibration energy of one of the symmetrical structures to betransferred to the other, and vice versa, both for the vibratorymovement along Ox and for the vibratory movement along Oy, since thecoupling structure is connected to that one of the two moving elementsthat vibrates both along Ox and along Oy.

In what follows, the coupling structure will therefore be referred to asthe “outer frame”, the first moving element will be referred to as the“intermediate frame” or the “inertial intermediate frame”, and thesecond moving element will be referred to as the “moving mass”, thismass being surrounded by the inertial intermediate frame.

The gyroscope according to the invention, produced by micromachining athin flat wafer (preferably a silicon wafer), therefore preferablycomprises, in the plane of the wafer, moving elements and anchoringregions, the moving elements comprising two flat moving masses, a flatmoving intermediate frame around each mass, and a coupling structurethat connects the two moving intermediate frames in order to allowmechanical energy to be transferred between the two intermediate frames,an excitation structure associated with each moving intermediate framein order to excite vibration of the frame in the Ox direction in theplane of the wafer, and a detection structure associated with eachmoving mass, for detecting a movement of the mass in the Oy directionperpendicular to Ox and in the plane of the wafer. The mass is connectedto the intermediate frame that surrounds it via at least two (preferablyfour) first narrow and elongate flexure arms that exhibit high stiffness(resistance to elongation) in the Oy direction and low stiffness(resistance to flexure) in the Ox direction (in practice, they liemainly in the Oy direction and cannot stretch in this direction, whereasthey can flex in the Ox direction). The mass is also connected to ananchoring region via at least two (preferably four) second narrow andelongate flexure arms that exhibit high stiffness (resistance toelongation) in the Ox direction and low stiffness (resistance toflexure) in the Oy direction (in practice, they lie mainly in the Oxdirection and cannot stretch in this direction, whereas they can flex inthe Oy direction).

To obtain both high stiffness or resistance to elongation in onedirection and low stiffness in the direction perpendicular in the sameplane, all that is required is for the arms to have an overall length ofat least five times their width. These refer to relative stiffnesses,the absolute stiffness depending of course on the absolute dimensions ofthe arms.

Preferably, each first flexure arm is bent over in the form of a U andhas two elongate portions extending along the Oy direction, these twoportions being connected by a short linking element. In this case, it isdesirable to connect the short linking element of one of the first armsto the similar linking element of another first flexure arm, via acrosspiece elongated in the Ox direction, this crosspiece preventingasymmetric forces from being exerted on these two first flexure arms.

The structures for exciting the moving frames and the structures fordetecting the movement of the moving mass are preferably capacitivecombs comprising interdigitated electrodes. The fixed portion of a combis attached to an anchoring region, which also forms an electricalcontact for transmitting electrical signals to this fixed portion orfrom this fixed portion. The moving portion is attached to a movingelement, the intermediate frame for the excitation structure or themoving mass for the detection structure. The anchoring region or regionsof the moving mass form (or forms) an electrical contact on the movingportion of the comb, through the entire vibrating structure.

An additional interdigitated comb structure may be provided on themoving mass in order to adjust, by applying an adjustable DC voltagebetween the facing electrodes of the comb, the apparent stiffness of thesecond flexure arms in the Oy direction for the purpose of controllingthe natural resonant frequency of the structure. The apparent stiffnessis adjusted by introducing a negative electrostatic stiffness that isadded to the natural (positive) stiffness of the flexure arms.

Another interdigitated comb structure may be provided in order to exerta torque on the moving mass about an axis parallel to Oz. This structurepreferably comprises at least two combs in order to exert a torque in acontrolled direction and of controlled value. Said structure serves tocompensate for the phase quadrature bias encountered when deviationsfrom symmetry of the vibrating structure are generated as a result ofimperfect fabrication of the microgyroscope. We will return to thispoint later on.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein the preferred embodiments of the invention areshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription therof are regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent onreading the detailed description that follows, which is given withreference to the appended drawings in which:

FIG. 1 shows a top view of the general structure of the micromachinedgyroscope according to the invention;

FIG. 2 shows a variant of the structure, which additionally includes ameans of adjusting the vibration frequency of the moving mass by varyingthe stiffness of the flexure arms; and

FIG. 3 shows another variant of the structure, which additionallyincludes a means for compensating for any phase quadrature bias byadding a torsional prestress in one direction, which tends to reduce thebias.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the thin flat silicon wafer machined according to theinvention in order to make a gyroscope whose sensitive axis isperpendicular to the plane of the wafer (which is the plane of thefigure).

Silicon is chosen as preferred material, firstly for its mechanicalproperties and secondly for its high conductivity when it is dopedsufficiently with an appropriate impurity (boron in general in the caseof p-type silicon). Conductive silicon allows the electrical functionsof the gyroscope, and especially the excitation and detection functions,to be carried out. These functions are carried out by interdigitatedcapacitive combs supplied with electrical current or voltage. Thefingers of these combs, machined directly in the conductive silicon,serve as the plates of capacitors useful for the excitation anddetection functions.

The thickness of the starting silicon wafer is a few hundred microns.The wafer has, on the one hand, fixed anchoring zones formed in thisthickness and, on the other hand, the actual vibrating structure, whichis free relative to the anchoring regions and is formed over a smallerthickness, for example over a thickness of around sixty microns, and isisolated from the rest of the thickness of the wafer by a narrow gap.The silicon wafer is cut by micromachining, over this approximatelysixty-micron thickness, into the moving mass features, the moving frame,the coupling structure, the flexure arms and the interdigitated combsthat are desired.

The structure may be machined using, as starting substrate, asilicon-on-insulator substrate, but other methods are also possible. Asilicon-on-insulator substrate consists of a silicon substrate a fewhundred microns in thickness that carries, on its front face, a thinlayer of silicon oxide which is itself covered with a layer ofsingle-crystal silicon a few tens of microns in thickness. The machiningoperation consists in etching the silicon of the substrate via its frontface, into the desired surface features, by means of photoetchingtechniques commonly used in microelectronics, down to the oxide layer,with a selective etchant that etches the silicon without significantlyetching the oxide. The etching is stopped when the oxide layer is bared.This oxide layer is then removed by selective etching using anotheretchant so as to leave only the surface layer of single-crystal silicon,except in the anchoring regions where the oxide layer remains and formsa strong bond between the substrate and the surface layer ofsingle-crystal silicon. Machining via the front face defines the variouscutting operations for the moving parts. These are therefore the abovesurface features, anchoring regions and cutting operations for themoving parts, as will be seen in the figures.

The general structure of the gyroscope is a structure of the tuning forktype, that is to say a symmetrical structure comprising two movinginertial assemblies vibrating in phase opposition, these movingassemblies being connected together via a coupling structure serving totransmit, without any loss, from one assembly to the other, themechanical vibration energies of the two assemblies in order to renderthese vibrations in phase opposition. The symmetry of the structure is asymmetry with respect to an axis Al, with one moving assembly on eachside of this axis.

The coupling structure is formed by two rectangular outer frames 20 and20′ within which the moving inertial assemblies are located. The frames20 and 20′ are connected together via a short linking bar 22, which maybe regarded as being rigid. The linking bar 22 links the middle of oneside of the first frame to the middle of an adjacent side of the secondframe. It constitutes a center of symmetry of the entire structure andit is perpendicular to the axis Al and centered on this axis. The shortlinking bar 22 may be reinforced by two other short linking bars locatedon either side of the bar 22 and also centered on the axis Al. Theseother short bars, of greater or lesser distance from the bar 22, allowthe difference between the useful excitation and detection frequenciesof the microgyroscope to be adjusted (the detection frequency preferablybeing slightly different from the excitation frequency, and thefrequency difference representing the bandwidth of the gyroscope).

The outer frames 20 and 20′ of the coupling structure surround the twomoving assemblies, in principle over at least three sides, and they areconnected to these two assemblies preferably along sides perpendicularto the general axis of symmetry Al. The frames 20 and 20′ may(optionally) each be attached to an anchoring region 24, 24′ located inthe middle of one side opposite the side connected to the linking bar22. In this case, the frames 20 and 20′ each completely surround arespective inertial moving assembly. The central linking bar 22 and theother sides of the frames 20 and 20′ are not connected to fixedanchoring regions.

The interdigitated combs serving to make the inertial assemblies vibrateand to detect the movement resulting from the coriolis force are alsothemselves placed inside each of the outer frames 20 and 20′. In whatfollows, only the elements located inside the frame 20 will bedescribed, the structure for the other frame 20′ being strictlyidentical. The elements internal to the frame 20′ are denoted by thesame references as those of the frame 20, but with the addition of the“prime” suffix.

Each inertial assembly comprises a central moving inertial mass 30 andan intermediate inertial frame 50 that surrounds it and that istherefore located between the mass 30 and the outer frame 20.

The moving mass 30 can move only in the Oy direction (vertical axis inthe plane of the figure). The intermediate frame 50 can move along theOy axis and along the Ox axis perpendicular to Oy and also lying in theplane of the figure. The sensitive axis of the gyroscope is the Oz axisperpendicular to the plane of the wafer. A vibration of the inertialintermediate frame is excited in the Ox direction. When the gyroscoperotates about its sensitive axis Oz, the intermediate frame is made tovibrate along the Oy axis. This vibration along Oy is transmitted to themass 30, whereas the vibration along Ox is not transmitted. As will beseen, a vibration excitation structure is associated with theintermediate frame 50 and a vibration detection structure is associatedwith the inertial mass 30. The coupling structure, formed by the frames20, 20′ and the bar 22 which connects them, transmits the mechanicalvibration energy from the moving inertial assembly on one side of theaxis Al to the other, both for vibrations along Ox and vibrations alongOy as this coupling structure is connected directly to the intermediateframes that may vibrate both along Ox and along Oy.

The moving mass 30 is connected to fixed anchoring regions via at leasttwo flexure arms designed to permit the mass to move along Oy but toprevent any significant movement of the mass in the Ox direction. Thesearms are preferably located on either side of an axis of symmetry 32 ofthe mass, parallel to Ox. There are therefore two anchoring regions 34and 36 located on either side of the moving mass, these beingsymmetrical with respect to this axis of symmetry 32. In addition, theseregions are preferably located on another axis of symmetry 38 of themass, which axis is parallel to Oy. The flexure arms that connect themass 30 to the regions 34 and 36 are arms elongated in the Ox directionso as to have a high stiffness (a high resistance to elongation) in thatdirection. They are also very narrow, compared to their length, so as tohave a low stiffness in the Oy direction perpendicular to Ox. This lowstiffness allows the mass to move along Oy. There are preferably fourflexure arms rather than two, the mass being connected to the anchoringregion 34 via two arms 40 and 42 in line with each other and on eitherside of the region 34. The mass is also connected to the secondanchoring region 36 via two arms 44 and 46 in line with each other andon either side of the region 36.

In practice, as may be seen in FIG. 1, to save space in the Oy directionwithout significantly reducing the length of the mass in that direction,a cut is made in the mass around the anchoring zone. To maximize theflexibility of the flexure arms in the Oy direction by increasing theratio of the length to the width of these arms, each arm is connected onone side to a point near an end corner of the mass (the mass has inprinciple, a generally rectangular shape) and on the other side to theanchoring region located on the axis of symmetry 38. It should be notedthat it would also be possible to envisage giving the arms 40, 42, 44,46 a folded-over shape with two branches elongated in the Oy direction,the arms then being attached to the mass closest to the centralanchoring region.

It should also be noted that, rather than one central anchoring regionlocated in the middle of one side of the moving mass, there could be twoanchoring regions located more toward the end corners of the mass oneither side of the axis 38.

The moving intermediate frame 50 completely surrounds the mass 30. Themass 30 is connected to the intermediate frame 50 via at least twoflexure arms that have the particular feature of having a very highstiffness (very high resistance to elongation) in the Oy direction and alow stiffness in the Ox direction. These arms are elongated in the Oydirection and have a small width compared to their length, so as toexhibit this stiffness difference.

There are preferably four flexure arms of this type between the mass 30and the intermediate frame 50, the arms being each located in practiceat a corner of the moving mass if the latter is of generally rectangularshape. They are placed symmetrically on one side of the axis of symmetry32 of the mass (the axis parallel to Ox) and on the other side of theaxis of symmetry 38 (parallel to Oy).

These arms are denoted by the references 52, 54, 56 and 58 and theypreferably have a shape folded over in the form of a U in order toreduce their longitudinal dimension by a factor of two withoutsignificantly reducing their useful length, and therefore withoutsignificantly reducing the high ratio of their stiffness along Oy totheir stiffness along Ox. The two U-shaped folded over branches areelongated parallel to Oy and are connected together via a short linkingelement. However, it would be possible for the arms 52 to 58 not to befolded over but to lie completely along the Oy direction between theintermediate frame and the mass. By folding them over it is possible tosave space without significantly modifying the desired mechanicalcharacteristics.

If the arms are folded over as in FIG. 1, it is preferable for the shortlinking element (which connects the two branches of the U) of a firstarm 52 to also be connected to the corresponding short element of thearm 54 which is symmetrical with the arm 52 with respect to the axis 38.A crosspiece 60 is provided for this purpose, parallel to Ox, in orderto connect the bottom of the U of the linking arm 52 to the bottom ofthe U of the flexure arm 54, the arms 52 and 54 being symmetrical withrespect to the axis 38. A similar crosspiece 62, symmetrical with thecrosspiece 60 with respect to the axis 32, connect the symmetricalelements 56 and 58. These crosspieces 60 and 62, parallel to Ox,reinforce the symmetry of transmission of movement along Oy, imposed bythe moving intermediate frame 50, to the mass 30. They are not presentif the arms 52, 54, 56 and 58 do not have a folded-over shape as in thiscase the ends of the arms 52 and 54 could already be rigidly connectedvia the intermediate frame 50 itself.

As may be seen in FIG. 1, the elongate U-shaped folded-over form of theflexure arms between the moving frame 50 and the moving mass 30 isobtained by cutting into the moving frame and into the moving mass, butin general the flexure arms start approximately from an internal cornerof the intermediate frame toward a facing corner of the mass, even ifthe effective point of attachment of the arm on the frame or on the massis not exactly from this corner. The mass may be considered as beingoverall suspended via its four corners to the moving frame.

The moving intermediate frame 50, surrounded by the outer frame 20 ofthe coupling structure, is connected to this outer frame via shortlinking arms 64 on one side and short linking arms 66 on the other, thearms 64 being symmetrical to the arms 66 with respect to the axis ofsymmetry 32. The arms 64, like the arms 66, are distributed along oneside of the frame 50, this side being parallel to the Ox axis. Theseshort arms constitute practically rigid links through which the energyof vibration of Ox and Oy of the intermediate frame 50 (and of themoving mass 30) can pass into the coupling structure and therefore intothe second intermediate frame 50′ and the second moving mass 30′. In theexample shown, three short arms 64 are distributed along the side of theintermediate frame 50, three other short arms 66 are distributed alongthe opposite side.

There is no linking arm between the intermediate frame and the outercoupling frame along the sides parallel to the Oy axis.

The intermediate frame 50 is excited in vibration along Ox by a firststructure in the form of an interdigitated comb 70 that comprises afixed half-comb 72 attached to an anchoring region 74, and a movinghalf-comb 76 formed along a first side (parallel to Oy) of theintermediate frame 50. The teeth or fingers of the fixed half-comb 72,made of conductive silicon machined at the same time as the otherelements of the gyroscope, constitute the first plate of a capacitor andthe teeth or fingers of the moving half-comb 76, also made of conductivesilicon, constitute the second plate of this capacitor. Conventionally,the comb structure acts as an exciter, for exciting the movement of themoving portion thanks to the attractive forces that are exerted betweenthe facing fingers when a voltage is applied between the half-combs. Theexcitation voltage is an AC voltage in order to generate a vibrationmovement, and the frequency of this voltage is chosen to be close to themechanical resonance frequency of the structure. The excitation voltageis applied between the anchoring region 74 and one and/or the other ofthe anchoring regions 34 and 36. The fixed half-comb 72 is in directelectrical contact (via the conductive silicon substrate) with theanchoring region 74. The moving half-comb 76 is in contact with theanchoring regions 34 and 36 via the flexure arms 52 to 58, of the bodyof the moving mass, via the flexure arms 40 to 46 and via theintermediate frame 50, so that, when a voltage is applied between theanchoring region 74 and the anchoring regions 34 or 36, a voltage isapplied between the fixed portion and the moving portion of the comb 70.

The excitation movement generated in the intermediate frame 50 is alongthe Ox direction, the combs acting by modifying the mutual area ofoverlap of the intercalated fingers.

The microgyroscope preferably includes another interdigitated combstructure associated with the intermediate frame, symmetrical with thestructure 70 with respect to the axis 38. It comprises a fixed half-comb82, attached to an anchoring region 84, and a moving half-comb 86machined along one side of the intermediate frame 50. This structure mayserve as a detector for detecting the movement of the frame along Ox. Itis useful for servocontrol of the movement excited by the comb 70. Ingeneral, servocontrol is useful for adjusting the excitation frequencywith respect to the resonant frequency of the structure. The voltagesdetected by the structure 80 appear between the anchoring region 84 andthe anchoring regions 34 and 36 (or else the region 24).

At least one interdigitated comb is associated with the moving mass 30in order to detect the movement of the moving mass in the direction Oy.The orientation of these combs depends on the principle on which thedetection is based. If the detection is based on a measurement of thechanges in mutual overlap area between the fingers of the fixed andmoving half-combs, the comb for detecting the movement along Oy isplaced perpendicular to the excitation comb 70 (which also is based onchanges in overlap area). If the detection is however based on measuringthe changes in spacing between the fingers of the fixed half-comb andthe moving half-comb, the detection comb is placed parallel to theexcitation comb. Detection by the change in spacing between fingers ispreferred as it is more sensitive. The interdigitation of the combs isthen asymmetric at rest, the fingers of one half-comb not being exactlyin the middle of the gap between two fingers of the other half-comb,whereas a comb operating, (like the excitation comb) on the basis ofchanges in overlap area has the fingers of one half-comb in the middleof the gap between the fingers of the other half-comb.

This is the case shown in FIG. 1, in which the detection combs areplaced in the same general orientation as the combs 70 and 80, althoughthey are associated with a movement along Oy while the combs 70 and 80are associated with a movement (excitation or detection movement) alongOx.

In the example shown in FIG. 1, the moving mass is associated with twoidentical interdigitated combs 90 and 100 placed parallel to the axis ofsymmetry 38 and on either side of this axis. These combs act in the sameway, by detecting the movement of the mass along Oy, but as a variant itwill be possible to be limited to just a single comb placed at thecenter of the mass along the axis 38.

The comb 90 comprises a fixed half-comb 92 attached to an anchoringregion 94 and a moving half-comb 96 forming part of the moving massitself. The moving mass includes a cut-out for leaving space for thefixed comb 92 and for the anchoring region 94, and the edges of thiscut-out are cut in the form of fingers in order to constitute the movinghalf-comb 96 in which the fingers of the fixed half-comb areintercalated. In the example shown, the comb 90 is a double comb, thatis to say two sides of the cut-out in the mass 30 are provided withfingers, and the fixed half-comb 92 has fingers on either side of theanchoring region 94.

The interdigitated structure 100 is strictly symmetrical with thestructure 90 and is formed in another cut-out in the moving mass 30. Itcomprises a fixed half-comb 102, an anchoring region 104 and a movinghalf-comb 106.

In order to detect the movement along Oy, an electronic circuitassociated with this structure detects the frequency modulation of theelectrical voltages present between the anchoring region 94 and theanchoring regions 34 and 36, and/or between the region 104 and theregions 34 and 36. This modulation is due only to a movement of themoving mass along the Oy axis, since the mass can only move along thisaxis.

FIG. 2 shows an improvement of FIG. 1. At least one additionalinterdigitated comb associated with the moving mass has been provided.This comb allows the apparent stiffness of the flexure arms 40, 42, 44,46 to be electrically adjusted, by simply controlling the DC voltage,this stiffness adjustment having a direct consequence on the adjustmentof the natural vibration frequency along Oy in the presence of acoriolis force. This is because the natural mechanical resonance of themoving assemblies depends on the stiffness of the flexure arms thatoppose the vibration movement generated. By adjusting the stiffness, andtherefore the frequency, it is possible to compensate for the variationsin resonant frequency that might result from non-uniformities or defectsin fabrication. Any deviation between the actual frequency and theintended theoretical frequency may thus be compensated for.

With a comb supplied with DC voltage, and acting on the moving mass 30in order to exert a constant force in the Oy direction, it is possibleto exert, at rest, a stress on the flexure arms 40, 42, 44, 46. Thisstress tends to create a negative stiffness, of electrostatic origin,the absolute value of which is subtracted from the natural stiffness ofthese arms in the Oy direction.

The comb exerting this stress is a comb 110 oriented like the othercombs (along the general direction of Oy), and in this case it acts bychanging spacing between fingers of the half-combs (a comb with offsetfingers). A single central comb may suffice, but in the example shown inFIG. 2 two symmetrical combs (110 and 120) placed laterally on eitherside of the axis 38 have been provided. In the example shown, thesecombs are symmetrical with respect to the center of symmetry of themoving mass and the detection combs 90 and 100 are also symmetrical withrespect to this center, rather than with respect to the axis 38, but thesymmetry could be with respect to the axis 38 both for the combs 90 and100 and for the combs 110 and 120. The stiffness adjusting comb 110 islocated along the extension of the detection comb 90. It comprises afixed half-comb, an autonomous anchoring region (for an autonomouselectrical supply) and a moving half-comb, again consisting of fingersdirectly cut into the moving mass. The comb 120 is identical to the comb110.

Further improvement with other combs again associated with the movingmass is shown in FIG. 3. These combs have, here too, been designed,merely by way of example, to lie along the extension of the combs 90 and100 (which are then shorter than those in FIG. 1 or even those shown inFIG. 2). The additional combs are intended to exert, by application ofsuitable DC voltages to each of them, a force that twists the movingmass about its center of symmetry. This has the effect of modifying theorientation of the excitation movement with respect to the detectionmovement and of consequently modifying (in a direction tending tocompensate for it) the quadrature bias of the gyroscope.

The gyroscope bias is the non-zero signal value measured when theangular velocity of rotation of the gyroscope is zero. The quadraturebias results from movements along one axis, whereas a force is in factexerted on a perpendicular axis. This results from defects in therectangularity of the beam sections or of other asymmetry factors. Thisbias may be partly compensated for by exerting a certain torsion on themoving mass. This torsion is exerted for example by acting on twointerdigitated combs 130 and 140 that are located diagonally on eitherside of a center of symmetry of the moving mass 30. A DC voltage isapplied to each comb so as to exert a torque in the directionappropriate for compensating for the bias. The torque exists wheneverthe combs exert forces applied at different points and the directions ofwhich do not pass through the center of symmetry of the mass.

In the example shown in FIG. 3, two combs 130 and 140 have been providedfor exerting this torque, in addition to the detection combs 90 and 100and the frequency adjusting combs 110 and 120. However, a single comb130 would suffice provided that this comb exerts a force in a directionthat does not pass through the center of symmetry of the moving mass. Itwill also be understood that the combs 110 and 120, placed diagonally onthe moving mass and exerting forces in directions that do not passthrough the center of symmetry of the mass could serve both foradjusting the frequency and exerting the bias compensating torque. Inthis case, there is no need for the additional combs 130 and 140 sincethe forces exerted on the mass by the combs 110 and 120 are directed oneupward in the figure and the other downward in the figure (it will beassumed here that the combs are asymmetrically interdigitated and thatthey act on the spacing between the fingers rather than on the overlapof the fingers). Applying voltages of different amplitude to the combs110 and 120 creates both a torque and a resulting upward or downwardforce, the latter creating the desired negative stiffness. However, forreasons of symmetry and independence of the stiffness control and thetorsion control, a configuration such as that in FIG. 3 with combs 110and 120 specifically for stiffness adjustment and combs 130 and 140specifically for quadrature bias compensation will be preferred.

In the foregoing text, provision was made for the combs 90 to 140 to beplaced in cut-outs in the moving mass, but it would also be possible toenvisage them being placed along the edges of the moving mass withoutmodifying the principles which were explained above.

Thus, a microgyroscope has been described that can be easily producedfrom a silicon wafer in the plane of which was machined both two movinginertial assemblies and a mechanical coupling structure that surroundsthem, and in which each moving assembly was produced in the form of twoparts, namely a moving mass and a moving frame, the moving frame beingconnected to the coupling structure via rigid links and the moving massbeing connected to the frame, on one side, and to anchoring points, onthe other, via flexure arms that allow movement in the plane in only onedegree of freedom for the moving mass and in two degrees of freedom forthe frame. The two moving assemblies are mechanically coupled both forexcitation vibrations and for orthogonal vibrations resulting from thecoriolis force. The mechanical coupling does not take place via flexibleflexure arms but directly via rigid links between the moving frame andthe coupling structure (unlike in structures in which the couplingbetween moving assemblies takes place via flexure arms serving both forproviding suspension flexibility of the inertial assemblies and thecoupling between the two assemblies).

The gyroscope according to the invention may have very high qualityfactors both in excitation and in detection, thereby allowing thesensitivity of the gyroscope to be increased when identical excitationand detection frequencies are used.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfills all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill will be ableto affect various changes, substitutions of equivalents and variousother aspects of the invention as broadly disclosed herein. It istherefore intended that the protection granted hereon be limited only bythe definition contained in the appended claims and equivalents thereof.

1. A gyroscope with a vibrating structure, produced by micromachining athin flat wafer, comprising: two symmetrical moving assemblies coupledby a coupling structure that connects the two assemblies in order toallow mechanical vibration energy to be transferred between the twomoving assemblies, wherein each of the two symmetrical moving assembliescomprises two moving elements, an inertial first moving element beingconnected to the coupling structure and able to vibrate in two degreesof freedom in orthogonal directions Ox and Oy of the plane of the wafer,and a second moving element being connected, on one side, to the firstelement and, on the other side, to fixed anchoring regions via linkingmeans that allow the vibration movement of the first element in the Oydirection to be transmitted to the second element without permitting anymovement of the second element in the Ox direction, an excitationstructure being associated with the first moving element in order toexcite a vibration of the first element along Ox, and a movementdetection structure being associated with the second moving element inorder to detect a vibration of the second element along Oy, the firstmoving element being a rectangular intermediate frame surrounding thesecond moving element, denoted by a moving mass, and the couplingstructure comprising two outer frames, each of which surrounds theintermediate frame of a respective moving assembly.
 2. The gyroscope asclaimed in claim 1, wherein the moving mass is connected to theintermediate frame via at least two first narrow and elongate flexurearms that exhibit high resistance to elongation in the Oy direction andlow stiffness in the Ox direction, and the moving mass is connected toat least one anchoring region via at least two second narrow andelongate flexure arms that exhibit high resistance to elongation in theOx direction and low stiffness in the Oy direction.
 3. The gyroscope asclaimed in claim 2, wherein each first flexure arm is bent over in aform of a U and has two elongate portions extending along the Oydirection, the two portions being connected by a short linking element.4. The gyroscope as claimed in claim 3, wherein the short linkingelement of one of the first arms is connected to a similar linkingelement of another first arm via a crosspiece elongated in the Oxdirection.
 5. The gyroscope as claimed in claim 1, wherein the couplingstructure is connected to the first moving element of each assembly viashort rigid links.
 6. The gyroscope as claimed in claim 1, wherein thecoupling structure comprises, around each moving assembly, an outerframe and a short linking bar between the outer frames.
 7. The gyroscopeas claimed in claim 1, wherein the structure for exciting the firstmoving element is a capacitive comb with interdigitated electrodes,which is machined in a thin flat wafer.
 8. The gyroscope as claimed inclaim 1, wherein the structure for detecting movement of the secondmoving element is a capacitive comb with interdigitated electrodes,which is machined in a thin flat wafer.
 9. The gyroscope as claimed inclaim 1, wherein it includes at least one interdigitated associated withthe second moving element of each assembly, for detecting the movementof the latter along Oy, and at least one additional interdigitated comb,electrically separated from the first comb, for exerting an adjustableforce on the second moving element by applying an adjustable voltage tothe additional comb, allowing the natural resonant frequency of themoving assembly to be modified.
 10. The gyroscope as claimed in claim 1,wherein the gyroscope includes at least one interdigitated combassociated with the second moving element of each assembly, fordetecting the movement of the latter along Oy, and at least oneadditional interdigitated comb associated with the second moving elementof each assembly, in order to exert, by applying an adjustable voltageto the additional comb, an adjustable torque on the second movingelement about an axis Oz perpendicular to Ox and Oy.
 11. The gyroscopeas claimed in claim 1, wherein the gyroscope includes at least threeinterdigitated combs associated with the second moving element, thefirst one for detecting a movement of the second moving assembly alongOy, the second one for adjusting a detection frequency and the third onefor exerting an adjustable torque on the second moving element.